Tissue graft

ABSTRACT

The present invention relates to a method of preparing a tissue graft material. The invention also relates to a multipurpose tissue graft material and to methods of using same as a replacement for vascular and non-vascular tissue.

The present application claims priority from Provisional Application No.60/178,632, filed Jan. 28, 2000, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of preparing a tissue graftmaterial. The invention also relates to a multipurpose tissue graftmaterial and to methods of using same as a replacement for vascular andnon-vascular tissue.

BACKGROUND

In general, biological tissues have a better functional performance thanequivalent synthetic devices when used as a body implant. Tissue graftsare presently largely limited to autologus and allograft tissues thathave inherent supply constraints and logistic concerns of harvest,transportation and serologies. Accordingly, there is a need foradditional sources of biological tissue grafts. Animal tissues representsuch a source. Animal tissues can be relatively easily obtained fromslaughterhouses in large quantities. Prior to use, however, thesetissues must be treated to remove antigenic proteins that elicit arejection response by the host following implantation.

Removal of antigenic proteins can be achieved by processing the donortissue in a manner such that the cellular component of the donor tissueis removed. Many antigenic proteins are present on cellular membranes.Therefore, removal of cells also removes these proteins. Afterdecellularization, the tissue can be packaged and sterilized for use asa biological graft. Grafts can be implanted into humans and otheranimals to repair, augment or replace natural structures, systems orexisting prosthetic devices. These include but are not limited to,cardiovascular, vascular, urogenital, neurological, gastrointestinal andorthopedic systems. Grafts can also be used to provide hemodialysisaccess.

The present invention provides a method of processing animal tissue soas to render it suitable for implantation into a human (or non-human)host. The invention also provides a method for processing human tissuefor use as an allograft implant.

SUMMARY OF THE INVENTION

The present invention relates to a method of preparing a tissue graftmaterial and to the resulting multipurpose graft material. The inventionalso relates to a method of using the tissue graft as a replacement forvascular or non-vascular tissue.

Objects and advantages of the present invention will be clear from thedescription that follows.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of preparing an animal orhuman tissue in a manner so as to render it suitable for use in vascularand non-vascular graft applications. Tissue prepared in accordance withthe method of the present invention exhibits physical and biologicalproperties that render it particularly well adapted for tissue graftapplications.

Tissue suitable for use in the present invention can be obtained fromhuman cadavers or from bovine, porcine or other animal, for example,under abattoir conditions. Tissue can be transported to the point oftissue preparation under conditions necessary to keep the tissue at adesired temperature. Tissues can be transported, for example, immersedin a physiological salt solution and, upon arrival, inspected, washed,for example, in a physiological salt solution, and cleaned (dissected)free of unwanted adherent material, such as connective tissue and fat.

In a preferred embodiment, an isolated ureter is the tissue graftmaterial. However, other tissues can be used including arteries, veins,tendons, heart valves, fascia lata, pericardium and nerves.

After collection and dissection, the transplant tissue isadvantageously, first washed, for example, with phosphate bufferedsaline (PBS), to reduce microbial bioburden. The tissue is thenincubated (e.g., at about 37° C. for about 18 hours) in a solutioncontaining one or more antimicrobial agents, for example, an antibioticor an antifungal agent, or mixture thereof, to further reduce thebioburden. Preferred antibiotics include amakacin, lincomycin,cefotaxime, vancomycin, rifampin, diflucan and amphotericin B.Advantageously, a mixture of these antibiotics is used. The tissue canthen be cryopreserved for further processing at a later time orimmediately subjected to decellularization.

Decellularization is preferably accomplished by incubating the tissue ina solution effective to lyse native cells in the tissue. Advantageously,the tissue is incubated (e.g., at about 37° C.) in sterile water (forexample, for about 4 hours in the case of ureters), however an aqueoushypotonic buffer or low ionic strength buffer can also be used. Ifdesired, the decellularizing solution can include other agents, such asprotease inhibitors ((e.g., chelators such as EDTA)).

After decellularization, the resulting tissue matrix is treated with anenzyme (e.g., nuclease) cocktail to degrade nuclear material. Nucleasesthat can be used for digestion of native cell DNA and RNA include bothexonucleases and endonucleases. Other nucleases are suitable for use inthis step of the process and are commercially available. For example, acocktail can be used comprising DNAse I (SIGMA Chemical Company, St.Louis, Mo.) and RNAse A (SIGMA Chemical Company, St. Louis, Mo.).

Preferably, the nucleases are present in a buffer solution that containsmagnesium and calcium salts (e.g., chloride salts). The ionicconcentration and pH of the buffered solution, the treatment temperatureand the length of treatment are selected to assure the desired level ofeffective nuclease activity. In the case of ureters, the buffer ispreferably a Tris buffer at pH 7.6. Preferably, the nuclease cocktailcontains about 0.1 μg/ml to 50 μg/ml, preferably 17 μg/ml, of DNAse I,and about 0.1 μg/ml to 50 μg/ml, preferably 17 μg/ml, of RNAse A. Thenuclease treatment can be effected at, for example, about 20° C. toabout 38° C., preferably at about 37° C., for about 1 to 36 hours. Inthe case of ureters, nuclease treatment for about 19 hours is typicallysufficient.

Subsequent to decellularization and nuclease treatment, the resultanttissue matrix can be treated (washed) to assure removal of cell debriswhich may include cellular protein, cellular lipids, and cellularnucleic acid, as well as extracellular debris, such as extracellularsoluble proteins, lipids and proteoglycans. Removal of cellular andextracellular debris reduces the likelihood of the transplant tissuematrix eliciting an adverse immune response from the recipient uponimplant. For example, the tissue can be incubated in a buffer (e.g.,PBS) or in a detergent solution such as a solution of Triton X-100 inwater. The composition of the solution, and the conditions under whichit is applied to the tissue matrix can be selected to diminish oreliminate the activity of the nuclease utilized during nucleaseprocessing and to remove cell debris. The process can include incubationat a temperature of between about 2° C. and 42° C., with 37° C. beingpreferred. The tissue matrix can be incubated in the detergent solutionfor up to 7 days, about 24 hours being sufficient in the case of aureter matrix. When buffer is used rather than a detergent solution, thetissue matrix can be incubated for up to 30 days, about 14 days beingsufficient in the case of a ureter matrix.

If used, the detergent solution can be washed out of the tissue matrixusing multiple washes in a sterile aqueous solution (e.g., water).Optimum wash number and times can be readily determined, however, about4 30 minute washes are preferred in the case of ureter matrices.

After washing, the tissue matrix can then be packaged, sterilized and/orstored prior to implantation. Advantageously, packaged tissue matrix ismaintained in a non-frozen state, preferably at a temperature between 0°C. and 40° C., more preferably, between 0° C. and 20° C., mostpreferably between 2° C. and 8° C. up to and during sterilization using,for example, the approach used in Example 11. After sterilization, thetissue matrix can be maintained at room temperature. If desirable, thetissue matrix can be cryopreserved before or after sterilization forlater use. Techniques of cryopreservation of tissue are well known inthe art. Brockbank, K. G. M. Basic Principles of ViableTissuePreservation. In: Transplantation Techniques and use ofCryopreserverd Allograft Cardiac Valves and Vasular Tissue. D. R. Clarke(ed.), Adams Publishing Group, Ltd., Boston. pp 9-23, discussescryopreservation of tissues and organs and is hereby incorporated byreference.

Tissue matrices of the invention, whether or not previouslycryopreserved, can be sterilized using art recognized sterilizationtechniques. Advantageously, sterilization is effected using gammairradiation at a dose of between 10 kGy and 100 kGy, preferably between20 kGy and 40 kGy, more preferably between 25 kGy and 40 kGy.Alternative modes of sterilization include iodine peracetic acidtreatment or electron beam. After sterilization, the tissue matrix canbe stored frozen or unfrozen prior to implantation. If storedcryopreserved, for example, in liquid nitrogen, the tissue matrix isstable for at least 5 years. Prior to using a frozen tissue matrix, thematrix is thawed using a protocol designed to elute cryoprotectantsolutions. For example, the matrix can be thawed rapidly to 4° C. in awaterbath at a temperature of 37-42° C. The matrix can then be quicklytransferred to a growth medium such as Dulbeccos' Modified Eagles Medium(DMEM) containing mannitol (e.g., at about 0.5%). Mannitol and residualcryoprotectants can be removed by serial dilution (washing) with 0.5,0.25 and 0.0M solutions of mannitol in DMEM. Following the washes, thetissue is ready to be used. For tissues not stored frozen, alternatewashing protocols can be used, for example, washing in PBS or DMEM.Art-recognized implantation procedures can be used and the procedureselected is dependent on the tissue matrix used and site ofimplantation.

The tissue matrix resulting from the above-described process,particularly a ureter matrix, can be used as a conduit (tubular) graft.For example, a ureter matrix can be used as a vascular graft, nerveguide, or replacement for any tubular structure, including a ureter.When used as a conduit graft, the diameter of the graft should generallybe about the same as the diameter of the native structure. The grafts ofthe invention demonstrate favorable characteristics as hemodialysisaccess grafts.

For use in other graft applications, a conduit graft (e.g., uretertissue matrix) resulting from the above process can be cutlongitudinally and rolled out to form a patch of tissue. The entiredecellularization/nuclease treatment procedure described above can becarried out on patches of tissue (e.g., ureter tissue) prepared bycutting the segment longitudinally and unrolling it to form a pre-graftpatch. The prepared graft patches can be utilized, for example, as askin graft material or for repair of other body tissue defects lendingthemselves to surgical application of a tissue graft patch having thephysical and functional characteristics of the present graftcomposition.

The tissue matrix of the present invention acts as a scaffold forspontaneous repopulation by host cells in vivo leading to tissuereconstruction and stabilization. The result is a fully functional,non-immunogenic, viable construct containing autologous cells expressingcontractile proteins. The better patency rate and lack of infection seenin the present grafts may be attributable to the early incorporation,recellularization and remodeling of the matrix with host cells.

Certain aspects of the present invention are described in greater detailin the non-limiting Examples that follow.

EXAMPLE 1 Implantation of Decellularized Bovine Ureter as a PeripheralVascular Graft in the Dog

Methods

Fresh bovine ureters were collected from slaughterhouses within 2 hoursof death and shipped to the processing facility at 4° C. in a solutionof phosphate buffered saline (PBS) to prevent tissue degradation intransit. Upon receipt of the tissue, a gross tissue inspection was madeand ureters of 4 mm outside diameter and 30 to 40 cm in length wereselected for processing. Selected ureters were dissected using sterileinstruments to remove unwanted adherent material such as connectivetissue and fat. Ureters were then placed in 300 ml capcity polypropylenecontainers and washed four times with 250 ml sterile PBS to reducemicrobial bioburden. Ureters were then taken through steps to remove thetissue cell and antigen content.

Decellularization was initiated by incubation in a cocktail ofantibiotics and antimycotic agents which consisted of a solution ofantibiotics. This mixture contained the following: amakacin (34 μg/ml),lincomycin (160 μg/ml), cefotaxime (181 μg/ml), vancomycin (136 μg/ml),rifampin (82 μg/ml), diflucan (120 μg/ml) and amphotericin B (0.5μg/ml). Incubation was for 18 hours in a shaking incubator at 37° C. Forcell lysis, the antibiotic solution was replaced with 250 ml of sterilewater and incubation was allowed to proceed for 4 hours in a shakingwaterbath at 37° C. This was followed by incubation for 19 hours at 37°C. in an enzyme cocktail to degrade the nuclear material now exposed bylysing the cell organelles. This cocktail contained DNAse I (47 KunitzU/ml) and RNAse A (1 Kunitz U/ml) in a solution containing magnesiumchloride (1 μg/ml)and calcium chloride (3 μg/ml) and buffered usingTris[hydroxymethyl]aminomethane hydrochloride (50 μg/ml) at pH 7.6. (TheDNAse I and RNAse A were obtained from Sigma Chemical Company (D-5025and R-5000) and both were used at a concentration of 17 μg/ml.)Subsequently, the tissue was placed in a 3.5 mM solution of Triton X-100detergent in sterile water to remove cell debris. This incubation wascarried out for 24 hours at 37° C. in a shaking waterbath. The detergentsolution was then washed out of the tissue by four washes in sterilewater at 37° C. for 30 minutes each. After these washes, the resultingmatrix was packaged, cryopreserved, sterilized and placed in storageprior to use as a vascular graft.

For preservation, the tissue was packaged in sterile packages containingDulbeccos' Modified Eagles Medium solution (DMEM) and 10% dimethylsulfoxide (DMSO) with 10% fetal bovine serum. Cryopreservation wasperformed using a controlled rate freezer to reduce the packagetemperature to −80° C. at 0.5° C. per minute. When the tissuetemperature had reached −80° C. each package was removed and placed inliquid nitrogen at −196° C. for long term storage. Sterilization oftissue was performed in the frozen state using a 25-30 kGy dose of gammaradiation. Following sterilization, the tissue was stored at −196° C. inliquid nitrogen until use.

Prior to implantation, the tissue matrix was thawed to remove thecryoprotectant solution from the tissue. The grafts were thawed rapidlyto 4° C. in a waterbath at a temperature of 37-42° C. Tissue was quicklytransferred to DMEM containing 0.5% mannitol. Mannitol and residualcryoprotectants were removed by dilution with 0.5, 0.25 and 0.0Msolutions of mannitol in DMEM. Following these washes, implantation ofacellular bovine conduit (that is, the resulting tubular tissue matrix)was performed as an end-to-side interpositional graft in the left andright carotid and left femoral arteries of an adult mongrel dog. Thegraft lengths were between 9 and 12 cm and the internal diameter of thegrafts was between 5 and 10 mm. Implantation was made using standardsurgical techniques for vascular graft implantation in these positions.An oral anticoagulant regimen of 325 mg Aspirin and 75 mg Persantin wasadministered daily to the animal beginning two days prior to thesurgical procedure.

At two weeks and at four weeks after surgery, arteriograms wereperformed to determine the patency of the implanted grafts. The animalwas sacrificed and the grafts explanted immediately following the secondarteriogram at four weeks after the surgery. Explanted grafts wereevaluated for patency and gross appearance and further examinedhistologically to determine graft microscopic integrity.

Results

During the four weeks duration of the study, the animal behaved normallyand there were no complications following surgery. At two weeks and atfour weeks after surgery, all three bovine ureter grafts were determinedto be fully patent on angiographic examination. At four weeks aftersurgery, gross analysis of the explanted graft tissue indicated there tohave been a healing response that had stabilized the grafts into thesurgical site and the patency of all the grafts was confirmed byobserving flow through the graft prior to placing the grafts in formalinfor fixation. After fixation each graft was cut into seven separatesamples for histological analysis. Samples were taken from the nativeartery at both proximal and distal ends away from the graft. Sections ofthe proximal and distal anastamosis sites were taken along with theproximal, middle and distal portion of each graft. Following processing,paraffin embedding and sectioning, graft samples were stained using astandard hematoxylin and eosin stain. Microscopic analysis revealed thegrafts to be structurally intact. The matrix of the bovine ureter hadbegun to become re-vitalized through the movement of cellular componentsfrom the outer edges of the surgical area. Through this remodeling thegrafts were taking on the appearance of natural arterial blood carryingconduits.

EXAMPLE 2 Implantation of Decellularized Porcine Ureter as a PeripheralVascular Graft in the Dog

Methods

Porcine ureter tissue was collected and prepared and preserved exactlyas described in Example 1 with the exception that the dimensions oftissues selected for processing were 3 mm in internal diameter and 25 cmin length. Implantation of treated porcine ureters was made in an adultmongrel dog as an end-to-end interpositional vascular graft in the leftand right femoral arteries and in the left and right carotid arteries.All grafts were examined by arteriogram two weeks after surgery. Theanimal was sacrificed four weeks after surgery and the grafts wereexplanted for gross examination and histological evaluation ofperformance. Histology samples were taken and stained as described inExample 1.

Results

Arteriograms performed two weeks after implantation showed the grafts tobe patent. On explantation, gross examination indicated the grafts to befully patent and difficult to distinguish from the native blood vessel.Histological examination showed the grafts to have become partlyrecellularized with spindle-shaped cells. This recellularization likelyrepresents the first stages of remodeling into a fully-functionalblood-carrying conduit that would be indiscernible from native tissue.

EXAMPLE 3 Implantation of Decellularized Bovine Uterine Artery as aPeripheral Vascular Graft in the Dog

Methods

Bovine uterine artery tissue was collected and prepared and preservedexactly as described in Example 1. Implantation of treated bovineuterine artery was made in an adult mongrel dog as an end to sideinterpositional graft in the carotid artery. After 4 weeks ofimplantation, the grafts were explanted and taken for histologicalanalysis.

Results

At four weeks after implantation, the grafts were patent. Histologyindicated these grafts to have begun to take on cells from the hostanimal.

EXAMPLE 4 Implantation of Decellularized Bovine Gastric Artery as aPeripheral Vascular Graft in the Dog

Methods

Bovine gastric artery tissue was collected and prepared and preservedexactly as described in Example 1. Implantation of treated bovineuterine artery was made in an adult mongrel dog as an end to sideinterpositional graft in the carotid artery. After 4 weeks ofimplantation, the grafts were explanted and taken for histologicalanalysis.

Results

At four weeks after implantation, the grafts were patent. Histologyindicated these grafts to have begun to take on cells from the hostanimal.

EXAMPLE 5 Determination of Burst Strength Characteristics ofDecellularized Bovine Ureter

Methods

Three segments of bovine ureter tissue were collected and prepared andpreserved exactly as described in Example 1. Using compressed nitrogengas, the pressure required to burst the graft was determined by slowlyincreasing the head-pressure of nitrogen applied to the graft. The gaswas contained within the graft using standard high-pressure pipefittings and cable ties. Each graft segment was tested in duplicate andaverage burst strength for each graft was calculated.

Results

The three grafts were found to burst at 3361, 2456 and 2327 mm Hg, theaverage being 2715 mm Hg. This magnitude represents a burst strength ofaround 1.5 times that of the human fresh saphenous vein which iscommonly used in bypass surgical procedures.

EXAMPLE 6 Determination of Burst Strength Characteristics ofDecellularized Porcine Ureter

Methods

Six segments of porcine ureter tissue were collected and prepared andpreserved exactly as described in Example 2. Using compressed nitrogengas, the pressure required to burst the graft was determined by slowlyincreasing the head-pressure of nitrogen applied to the graft. The gaswas contained within the graft using standard high-pressure pipefittings and cable ties. Each graft segment was tested in duplicate andthe average burst strength for each graft was determined.

Results

The six grafts were found to burst at 2068, 5171, 6722, 6722, 5688 and3620 mm Hg, the average being 4999 mm Hg. This value represents a burststrength of almost 3 times that of human fresh saphenous vein which iscommonly used in bypass surgical procedures.

EXAMPLE 7 In Vitro Recellularization of Decellularized Bovine Ureterwith Vascular Conduit Cells

Methods

Three segments of bovine ureter tissue were collected and prepared andpreserved exactly as described in Example 1. Each piece of tissue wasplaced into a 75 cc tissue culture flask containing DMEM andsupplemented with 10% fetal bovine serum. Each graft was seeded withendothelial cells or smooth muscle cells to enable cell growth into thegraft segments. Cultures were fed using fresh serum-supplemented DMEMtwo times each week for 4 weeks. After 4 weeks, the tissues wereextracted from the culture system and examined using histologicalsectioning of the tissue and H&E staining.

Results

After four weeks of cell culture, endothelial cells were observedgrowing on the surface of graft tissue but not internally. In addition,smooth muscle cells were found growing on the surface of the grafts andin the wall of the graft material once they could gain access on thesurface of the graft.

EXAMPLE 8 Tissue Graft Derived from Ureter as Aortic Graft in the Dog

Methods

Bovine ureters were used to provide the conduit matrix for the vasculartissue graft. These tissues are available in lengths and diameterssuitable for a number of vascular applications and they do not containvalves in the lumen or possess tributaries that require ligation.Ureters were obtained from U.S Department of Agriculture approvedslaughterhouses. The tissues were washed in physiological salt solutionand transported for tissue preparation on ice within 24 hours ofharvest. Ureters were first dissected free of adherent connective tissueand fat and only segments with a 6 mm internal diameter were taken forfurther processing.

Initial processing consisted of bioburden reduction using a solution ofmultiple antibiotics as described in Example 1. Removal of greater than95% of all cellular material was achieved in several steps. First,incubation in sterile water produced hypotonic cell lysis. The resultingtissue matrix was then equilibrated in buffer (PBS) and treated with asolution containing ribonuclease and deoxyribonuclease (see Example 1).An isotonic washout over several days completed the cellular proteinremoval. Removal of cellular debris was monitored using hematoxylin andeosin staining of histological sections. Tissue matrices were thensterilized by gamma irradiation (25 kGy to 40 kGy) prior to use andanalysis of sterility was carried out on each processing batch.

Eight mongrel dogs weighing 50 to 60 lb were anesthetized with sodiumthiopental, endotracheally intubated and placed on inhaled isoflurane.The abdomens were prepared and draped in sterile fashion. A midlineincision was made and the abdominal aorta distal to the renal arterieswas isolated in each dog. Vascular conduits were prepared by washing thetissue matrix in 100 ml of sterile HEPES-buffered Dulbecco's ModifiedEagle Medium and a segment approximately 6 cm in length and 6 mm ininternal diameter was inserted as an aortic interposition graft usinginterrupted prolene sutures to construct proximal and distal, end toend, anastomoses. All animals received 325 mg aspirin and 75 mgdipyridamole p.o. daily for 2 days prior to, and for 14 days following,surgery.

Patency and structural stability were observed with angiographicexamination following surgery every 6 weeks in the longer survivors andonce immediately prior to euthanasia. Two animals were sacrificed at 3weeks, 3 at 6 weeks, and 1 animal at 13 weeks after surgery. The 2remaining animals were last evaluated at 43 weeks and are still living.After sacrifice, grafts were removed in bloc incorporating proximal anddistal anastamoses inspected grossly and processed for histologicalanalysis.

Following harvest, grafts were fixed in 10% buffered formaldehydesolution. The whole of the graft along with anastamotic sites andproximal and distal native aorta was divided into 7 tissue segments andplaced in paraffin blocks for processing. Hematoxylin and eosin-stainedsections of these tissues were examined and immunohistochemical analysiswas carried out using specific antibiotics to identify the presence ofsmooth muscle α-actin (α-SMA), desmin and vimentin contractilefilaments.

Results

After processing, vascular tissue grafts prepared from bovine uretershowed removal of greater than 95% of bovine cellular material. Theremainder consisted of cellular debris and not intact cells. Conduitgraft sterility and pyrogen levels of below 20 endotoxin units weredemonstrated. Implantation of these interposition grafts into the canineinfrarenal aorta was uncomplicated and handling properties of the graftswere similar to normal vascular tissue.

Arteriograms performed on each of the dogs indicated grafts to be fullyfunctional over the 43-week implant period without the appearance ofdilation or stenosis. Gross evaluation of all explanted grafts after 3,6 and 13-weeks of implantation confirmed fully patent grafts. Histologicexamination showed a healing response around the graft adventitia withrecellularization of the media. A layer of cells on the lumenal surfaceresembled endothelium. All cells found in the graft were presumed tohave originated from the host because the original graft material wasacellular. The extent of medial recellularization was approximately 20%at 3-weeks, 30% at 6-weeks and 50% at 13-weeks. Revitalization of thegraft media appeared to occur from the adventitial area towards thelumen and as recellularization progressed, there was circumferentialorganization of cells growing perpendicular to the flow of blood in theconduit.

Analysis of anastomotic sites showed intimal hyperplasia to be minimaland cellular overgrowth was evident at the suture-line creating a smoothtransition from native aorta to graft. Histologically, there was noevidence of hyperplastic reaction narrowing the lumen in the graftexplanted after 13-weeks. Also, narrowing was not observedangiographically up to 43-weeks after implantation.

Immunohistochemistry staining was used to identify the type of cellspresent in the recellularized grafts. The proportion of cells expressingsmooth muscle contractile proteins were demonstrated using stainscontaining antibodies to α-SMA, desmin and vimentin. A very largepercentage of medial cells at 3, 6 and 13 weeks, were α-SMA positive.Vimentin was also commonly expressed by α-SMA positive cells. Desminpositive cells were less abundant but present in a sub-population. Mostof the cells present in the grafts stained positive for at least one ofthese contractile proteins. As no intact cells were present in the graftconduits prior to implant, all cell-specific immunostaining demonstablefor α-SMA, desmin and vimentin was present on cells that had originatedfrom the host.

EXAMPLE 9 Use of Tissue Graft Derived from Ureter as an Arterio-venousFistula in the Dog

Methods

Nine segments of treated bovine ureter tissue graft conduit, prepared asdescribed in Example 9, (20 cm×6 mm ID) were implanted as arteriovenousgrafts in the carotid artery (CA) and jugular vein (JV) (n=5), or in thefemoral artery (FA) and femoral vein (FV) (n=4) in 6 adult dogs. Acontrol group of 7 dogs received 11 (6 mm ID) polytetrafluoroethylene(PTFE) grafts (7 in the CA and JV, and 4 in the FA and FV). All graftswere matured for 14 days and then sham-accessed once weekly with two17-gauge hemodialysis needles. Routinely over a 6-month period, patencywas assessed and blood was drawn to monitor CBC and clotting factors.Histological analysis was performed in a sub-group of explanted graftsat 2, 4, 10 and 24 weeks.

Results

27% (3/11) of the PTFE grafts became infected, while none of the tissuegraft conduits prepared in accordance with the present invention becameinfected during the study. The patency rate of the tissue graft conduitwas 86% compared to 72% for the PTFE grafts. The white blood cell countwas not elevated in either group at 2 and 7 weeks and blood clottingfactors were also unchanged. The hemostasis times after sham sticking ofthe grafts was longer (mean 10 minutes) in the PTFE grafts compared tothe tissue graft conduits (mean 3 minutes). Histology at 10-weeks showedtissue graft conduits to have undergone recellularization of the tunicamedia with host spindle shaped cells as well as excellent incorporationinto surrounding tissues as evidenced by capillary ingrowth into thetunica adventitia. PTFE grafts showed no significant cellular ingrowthand an absence of luminal endothelium.

EXAMPLE 10 Packaging and Sterilization of Tissue Graft

Packaging

Tissue product is packaged in heat sealed clear polyester pouchescontaining phosphate buffered saline and stored at 40 centegrade for upto 7 days prior to sterilization.

Shipping

Three frozen 2 lb cold gel bricks are placed in the bottom of apre-chilled 19″×14.5″×22″ cardboard container insulated with 2″polyurethane foam. Two cardboard separators are placed on top of these.

Three cold 2 lb gel bricks are added followed by product load (1,100cubic inches).

Two filler bags are used as temperature indicators on which varioustemperature indicator strips are present, the filler bags are placedamong the samples. A seven-day mechanical temperature recorder is alsoplaced among the samples.

Three cold 2 lb gel bricks are place on top of the product load followedby a cardboard separator and three frozen 2 lb gel bricks. A foam plugis placed on top of the last layer of bricks and the box is closed.

The box is shipped to sterilization facility for sterilization by gammairradiation at 25-40 kGy. The box is then returned and product isunpacked and stored at room temperature until use. The total tissue timebetween packing and unpacking is advantageously less than 100 hours andthe temperature is maintained throughout this period at 2° C.-8° C.

Storage

The tissue graft can be stored at room temperature for 2 years.

All documents cited above are hereby incorporated in their entirety byreference.

One skilled in the art will appreciate from a reading of this disclosurethat various changes in form and detail can be made without departingfrom the true scope of the invention.

1. A tissue graft produced by a method consisting essentially of thesteps of: i) washing a starting tissue obtained from a human or animalureter with a bioburden reducing agent so that said starting tissue isdisinfected, ii) decellularizing the disinfected tissue resulting fromstep (i) with water or an aqueous hypotonic buffer that lyses cells ofsaid disinfected tissue so that a tissue matrix is formed, and iii)contacting said tissue matrix resulting from step (ii) with a nucleaseso that nucleic acid associated with said tissue matrix is degraded, andiv) washing said tissue matrix resulting from (iii) so that cellular orextracellular debris is removed and said tissue graft is produced. 2.The tissue graft according to claim 1 wherein, after step (iv), saidtissue matrix is sterilized.
 3. The tissue graft according to claim 1wherein said tissue graft is cryopreserved.
 4. The tissue graftaccording to claim 1 wherein said starting tissue is obtained from abovine or porcine ureter.